Movement characteristics of a vehicle depend on the operation of vehicle systems (e.g., the engine system, the steering system, the suspension system, the braking system, etc.) and also on external influences (e.g., the road gradient, etc.). The driver typically drives the vehicle in a predictable manner based on the recognition or anticipation of these movement characteristics. The driver typically adapts to characteristic changes of the vehicle. Detailed examples are listed as follows:
1) Characteristic Changes Related to Vehicle Systems:
In general, vehicle understeer can affect a vehicle as a turning curvature increases (i.e., where lateral acceleration increases). The understeer behavior depends upon vehicle weight distribution, suspension geometry (i.e., suspension geometrical structure), suspension kinematics (i.e., characteristic change when receiving forces), and tire characteristics, for example. Additional steering is more likely to be insufficient as the vehicle turns through a deeper curve, thereby creating an understeer characteristic.
2) Characteristic Change Caused by Travel Conditions:
Steering behavior changes as the vehicle speed changes. For instance, FIG. 7 includes a plurality of graphs showing a change of steering in accordance with vehicle speed where the vehicle travels on the same curved path at different speeds. Specifically, FIG. 7 shows changes in steering wheel angle, yaw rate, and lateral acceleration as changes of steering in accordance with vehicle speeds.
As seen in FIG. 7, the steering wheel angle, yaw rate, and lateral acceleration change with vehicle speed and are more vibratory at approximately a sprung natural frequency as the vehicle speed increases. Therefore, the driver operates the steering wheel and adapts to these changes.
Further, load balance between the front and rear wheels changes as the road gradient changes. As a result, steering characteristics change. For example, when the vehicle travels uphill, loads in the front wheels reduce, and understeering characteristics are more likely. On the other hand, when the vehicle travels downhill, loads in the front wheels increase, and oversteering characteristics are more likely.
In addition, the driver steers the vehicle based upon surrounding states of the vehicle. The driver selects a steering direction, and the driver typically reacts to surrounding states of the vehicle, correcting the steering direction according to vehicle movement feedback, states of the road surface, and the like.
Japanese Patent Abstract Publication No. 06-298112 relates to a four-wheel-steering system (Four-WS system). Responsiveness of the vehicle behavior (e.g., yaw rate) increases based upon steering wheel angle or steering angle of a tire calculated from the steering wheel angle. Accordingly, a steering angle of the front or rear tire is corrected for improving vehicle stability, or a detected yaw rate is incorporated into steering control methods.
Other conventional systems are designed such that when a driver performs a steering operation, a steering motor is operated to add an assist force to the force provided by the driver. Thus, the driver input force and the assist force creates torque in the steering shaft is supplied by the driver input and the assist force. Steering control systems have been designed to adjust the assist force when a vehicle spin or a vehicle drift is detected for improved stability.
As described above, a Four-WS system can improve vehicle stability. However, these systems can be prohibitively expensive. Furthermore, these systems can undesirably increase the weight of the vehicle.
In addition, steering control systems with steering motors can provide added stability by adjusting the assist forces. However, the assist forces are typically adjusted during vehicle spin or vehicle drift situations. These systems typically do not affect normal driving situations, and thus are of little use for affecting response to steering reaction and vehicle behavior.
Further, drivers often operate the vehicle largely according to what the driver sees in the surrounding environment. (It is estimated that 80% of information used for driving the vehicle is optical information.) As a result, driving characteristics can largely depend on how the environment changes visually. Also, when the road ahead of the vehicle is a curve or a hill, when a vehicle is parked ahead, or where the road ahead of the vehicle narrows, accelerations perceived by the driver will vary depending on the situation. The weather and/or visibility can also affect the driver's steering behavior. In addition, the driver's steering behavior may vary depending on whether the driver's line of sight is generally near the vehicle or generally in the distance.
More specifically, a driver is more likely to sense acceleration, decelerations, and yaw as the posture of the vehicle changes. For example, during deceleration, the front of the vehicle may pitch forward making it more likely for the driver to sense the deceleration. During acceleration, the vehicle may pitch rearward making it more likely for the driver to sense the acceleration. Further, a vehicle may roll during a turn causing the driver to more likely sense the turn.
In addition, relations between actual vehicle behavior during acceleration, deceleration, or turning, vehicle posture, and operation of vehicle components by the driver change due to vehicle characteristics or road environments. Therefore, a driver is required to perform a correction operation corresponding to the changed relation at proper timing.
Furthermore, while changing lanes, a driver expects lateral movement behavior of a vehicle rather than a rotational movement. This is because the driver views things generally straight ahead of the vehicle. Thus, the expectation degree to the turning is small. The Four-WS system steers the rear wheels in the same phase direction as the front wheels at high speeds. However, the vehicle behavior is not optimized based upon recognition of the environment of the vehicle, especially ahead of the vehicle.
In view of the above, there exists a need for a steering control system that overcomes the above-mentioned problems in the conventional art. The present invention addresses this need in the conventional art as well as other needs, which will become apparent to those skilled in the art from this disclosure.